Oscilloscopes, data recording devices and other test instrumentation use sampling circuits, which sample input signals at a predetermined sampling rate or sampling frequency. A typical sampling circuit includes a sampling switch, a capacitor connected between the sampling switch and ground, an amplifier, and a digitizer (e.g., analog to digital converter (ADC)), for example.
Conventional sampling circuits include narrow aperture sampling circuits. Aperture refers to the window during which an input signal is sampled. For a narrow aperture sampling circuit to have high bandwidth, the aperture must be very narrow in time. However, as the aperture becomes more narrow, gain of the narrow aperture sampling circuit becomes lower, resulting in very low gain when sampling high bandwidth input signals. From sampling theory, a perfect zero-aperture sampling circuit produces infinite bandwidth samples, but has zero gain. To compensate for the low gain at narrow apertures, the sampled signal may be amplified, but this results in increased noise. Therefore, a conventional narrow aperture sampling circuit generally produces an attenuated image of the original input signal, plus additional images of the original input signal symmetric about the sample frequency and its harmonics. A result of spreading the energy of the original input signal is that the gain of all the images goes down.
Conventional sampling circuits further include track and hold circuits and sample and hold circuits. Track and hold circuits, in track mode, must track to full bandwidth on a capacitive load, which is used to hold the sampled value of the input signal when a switch is turned off. There are inherent tradeoffs between achievable bandwidth, acquire time and hold time. Sample and hold circuits require the holding capacitor to be reset after a sample is digitized.